Personal vs corporate brand

With such availability to social media, it has never been so easy for employees to build their professional personal brand through avenues such as facebook, twitter, linkedin and even YouTube. Writing a blog about your industry can be a great way to put you on the corporate map. But does a tagline stating where you work require permission from the top? Can you attend an industry function without representing your company?

How do employers loosen the reigns and allow employees to build their own brand, when it's company time that's put them where they are and paid their wages? How can employees not afford to build a brand when time spent in jobs is increasingly shrinking?